Video displays such as computer monitors generate images using video signals received from a computer system, for example, a personal computer (or "PC"), or other source of video data. For many such displays, images are produced using data received from three video signals--red, green and blue--collectively referred to as RGB video data signals. In addition to the aforementioned video data signals, most video displays also receive two other signals--a horizontal synchronization (or "sync") signal and a vertical sync signal. The horizontal sync signal is used to synchronize the monitor to the video signal source. Specifically, the video signal source transmits a serial data stream to the video monitor which begins to scan from left to right across the screen using an electron beam. At the end of a line, a horizontal sync pulse indicates the end of the line. Upon receiving the horizontal sync pulse, the monitor will reposition the electron beam back to the left border of the screen and begin scanning to the right again. The vertical sync pulse, on the other hand, indicates to the video monitor to begin a new screen by repositioning the electron beam back to the top left corner of the screen.
Currently, there are three techniques by which these three video signals and two sync signals are transmitted from a video source to a video monitor. The first of these techniques may be seen by reference to FIG. 1a. Here, a video source 10 and a video monitor 12 are coupled together by first, second, third, fourth and fifth lines 14, 16, 18, 20 and 22, each of which respectively carries one of the R, G, B, horizontal sync (or "HSYNC") and vertical sync (or "VSYNC") signals from the video source 10 to the video monitor 12. While adequate for use, this configuration is considered disadvantageous in that it requires five shielded cables, together with associated connection circuitry, in order to convey the requisite signals. As a result, therefore, this configuration would be both more expensive to manufacture and less convenient for use in video applications where space is at a premium.
For these reasons, various solutions by which the five video signals are transmitted from the video source to a video monitor using a lesser number of cables have been proposed. A four line solution may be seen by reference to FIG. 1b. Here, a video source 24 is coupled to a video monitor 26 by first, second, third and fourth cables 28, 30, 32 and 34. As before, the first, second and third cables 28, 30 and 32 are used to transmit respective ones of the R, G and B video data signals. Here, however, the HSYNC and VSYNC signals are combined into a composite sync signal for transmission over a composite sync cable, for example, the fourth cable 34. While the inability of video monitors to distinguish between HSYNC and VSYNC pulses was initially considered to be an obstacle to this approach, this problem was solved relatively easily by varying the comparative duration of the HSYNC and VSYNC pulses. Thus, the video monitor 26 would distinguish between the HSYNC and VSYNC pulses based upon comparative duration, i.e., pulses having a duration in excess of a pre-selected value are classified as VSYNC pulses while pulses having a duration below the pre-selected value are classified as HSYNC pulses. While this configuration successfully reduced the number of required connections between the video source 24 and the video monitor 26 from 5 to 4 and achieved, therefore, considerable savings in both cost and consumption of space, it should be readily appreciated that further savings are possible if the number of required connections could be reduced still further.
A three line configuration is illustrated in FIG. 1c. As may now be seen, a video source 36 is coupled to a video monitor 38 by only first, second and third lines 40, 42 and 44. Similar to the systems disclosed in FIGS. 1a-b, the first and third cables 40 and 44 carry the R and B video data signals. Here, however, the second cable 42 carries both the G video data signal and the composite sync signal. Combining video data and sync signals on a single line is possible because of the characteristic of video data signals to periodically blank. Blanking intervals are those periods during which a video signal is inactive. For example, a video data signal blanks whenever the electron beam is positioned outside the active display area of the monitor, i.e., is positioned along the border or porch of the screen, or while the electron beam is repositioning itself for scanning a next line. or next screen.
While the three cable solution would appear to be the most desirable of the various configurations disclosed in FIGS. 1a-c, certain considerations have limited the use thereof. Specifically, to combine video and synchronization data into a single signal, synchronization data is inserted into the blanking intervals of the G video signal prior to transmission of the video/composite sync composite signal over the second line 42. Upon receipt by the video monitor 38, the synchronization data is stripped off before generation of an image thereby. So that the video monitor 38 can readily distinguish between the video data component and the sync component of the received video/synchronization composite signal, the video monitor 38 is instructed that the data component of the video/synchronization composite signal will always be positive while the sync component will always be negative. Such an instruction, however, presumes that all composite sync signals will be polarity insensitive, i.e., will always have the same polarity. If different sync signals had different polarities, important synchronization information could be lost when combining and/or separating the video and sync signals.
Unfortunately, this presumption is not always necessarily true. The polarity of synch pulses for RGB-type monitors is determined by the software drivers used in the computer's video boards. Thus, different computer systems may use sync signals having different polarities. In some platforms, for example, MS-DOS, HSYNC pulses are negative. In other platforms, for example, Windows, HSYNC pulses are positive. Thus, to switch between these two platforms, monitors, which are generally considered to be non-platform specific, must be sufficiently sophisticated to distinguish between the different types of sync signals and adjust their operations appropriately. If, however, the display system is configured as illustrated in FIG. 1c such that sync information is presumed negative and combined with a video signal prior to transmission from the video source 36 to the video monitor 38, the video monitor 38 would be unable to distinguish whether the sync signal, once separated from the video signal, should be positive or negative. For this reason, the three line solution illustrated in FIG. 1c is used infrequently. Instead, the four line (R, G, B, composite sync) solution illustrated in FIG. 1b, while requiring the use of additional cables and associated connection circuitry, is more commonly used since, by maintaining a dedicated sync line, the polarity information for synchronization signals is preserved.